Method for manufacturing hot-rolled coil, and method for shape-correction of hot-rolled coil

ABSTRACT

Embodiments include a method for manufacturing a hot-rolled coil and a method for correcting the shape of a hot-rolled coil. In one embodiment, the method for manufacturing the hot-rolled coil includes the steps of: reheating a steel slab comprising 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities; hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/KR2017/007870, filed Jul. 21, 2017,which claims the benefit of and priority to Korean Patent ApplicationNo. 10-2016-0093096 filed on Jul. 22, 2016, the entire content of eachbeing incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a hot-rolledcoil and a method for correcting the shape of a hot-rolled coil. Morespecifically, the present invention relates to a method formanufacturing a hot-rolled coil for preventing shape defects, which mayprevent shape defects from being caused by self-weight duringmanufacturing of the hot-rolled coil, and to a method for correcting theshape of a hot-rolled coil.

BACKGROUND

In recent years, ensuring light weight has been considered to be animportant factor in the development of automobile materials. This isintended to replace existing parts with high-strength materials, therebyultimately improving fuel efficiency. To this end, materials that arestructural materials for automobiles have been developed so as toimprove performance by adding alloying elements, including manganese(Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium (Ti) and thelike, and cold rolling and heat-treatment processes have been applied toensure the strength of steel.

Background art related to the present invention is disclosed in KoreanPatent Application Publication No. 1995-0016913 (published on Jul. 20,1995; entitled “Telescopic correction apparatus for hot-rolled coil”).

SUMMARY Technical Problem

One embodiment of the present invention is intended to provide a methodfor manufacturing a hot-rolled coil, which has an excellent effect ofpreventing the deformation of the hot-rolled coil.

Another embodiment of the present invention is intended to provide amethod for correcting the shape of a hot-rolled coil, which may preventdeterioration in the material and physical properties of the hot-rolledcoil.

Still another embodiment of the present invention is intended to providea method for correcting the shape of a hot-rolled coil, which mayprevent surface defects of the hot-rolled coil from occurring whencorrecting the shape by application of an external force.

Yet another embodiment of the present invention is intended to provide amethod for correcting the shape of a hot-rolled coil, which hasexcellent economic efficiency.

Technical Solution

One aspect of the present invention is directed to a method formanufacturing a hot-rolled coil. In one embodiment, the method formanufacturing the hot-rolled coil includes the steps of: reheating asteel slab including 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt %silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 but not morethan 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium(Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to0.04 wt % titanium (Ti), and the remainder being iron (Fe) and otherinevitable impurities; hot-rolling the steel slab at a finishing milldelivery temperature of 850° C. to 950° C., thereby forming a hot-rolledsheet; and cooling the hot-rolled sheet, followed by coiling at acoiling temperature of 700° C. or higher.

In one embodiment, the steel slab may include 0.21 to 0.37 wt % carbon(C), 0.1 to 0.4 wt % silicon (Si), 1.1 to 1.5 wt % manganese (Mn), morethan 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt %but not more than 0.02 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr),0.001 to 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and theremainder being iron (Fe) and other inevitable impurities.

In one embodiment, the steel slab may include 0.18 to 0.25 wt % carbon,0.3 to 0.5 wt % silicon (Si), 2 to 6.5 wt % manganese (Mn), more than 0wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % butnot more than 0.01 wt % sulfur (S), more than 0 wt % but not more than0.1 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt %boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron(Fe) and other inevitable impurities.

In one embodiment, the steel slab may include 0.5 to 0.56 wt % carbon(C), 0.1 to 0.3 wt % silicon (Si), 0.7 to 1 wt % manganese (Mn), morethan 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt %but not more than 0.01 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr),more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.02 wt% titanium (Ti), and the remainder being iron (Fe) and other inevitableimpurities.

Another aspect of the present invention is directed to a method forcorrecting the shape of a hot-rolled coil. In one embodiment, the methodfor correcting the shape of the hot-rolled coil includes the steps of:mounting the hot-rolled coil on a hanger forming the lower part of aC-hook; measuring the longest diameter of the hot-rolled coil using anouter diameter measuring means provided in the upper part of the C-hook;adjusting the longest diameter of the hot-rolled coil to beperpendicular to the C-hook by means of a driving roll provided on thehanger; and placing the C-hook, which has the hot-rolled coil mountedthereon, on a stand, followed by lifting, thereby correcting the shapeof the hot-rolled coil by self-weight.

Still another aspect of the present invention is directed to a methodfor correcting the shape of a hot-rolled coil. In one embodiment, themethod for correcting the shape of the hot-rolled coil includes thesteps of: mounting the hot-rolled coil on a hanger forming the lowerpart of a C-hook; measuring the longest diameter of the hot-rolled coilusing an outer diameter measuring means provided in the upper part ofthe C-hook; adjusting the longest diameter of the hot-rolled coil to beperpendicular to the C-hook by means of a driving roll provided on thelower hanger; and placing the C-hook, which has the hot-rolled coilmounted thereon, on a stand, followed by lifting, thereby correcting theshape of the hot-rolled coil by self-weight, wherein the hot-rolled coilis manufactured by a method including the steps of: reheating a steelslab including 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon(Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt %sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr),more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt% titanium (Ti), and the remainder being iron (Fe) and other inevitableimpurities; hot-rolling the steel slab at a finishing mill deliverytemperature of 850° C. to 950° C., thereby forming a hot-rolled sheet;and cooling the hot-rolled sheet, followed by coiling at a coilingtemperature of 700° C. or higher.

In one embodiment, the hot-rolled coil may include 0.21 to 0.37 wt %carbon (C), 0.1 to 0.4 wt % silicon (Si), 1.1 to 1.5 wt % manganese(Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), morethan 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 to 0.3 wt %chromium (Cr), 0.001 to 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium(Ti), and the remainder being iron (Fe) and other inevitable impurities.

In one embodiment, the hot-rolled sheet may be cooled and coiled at acoiling temperature of 700° C. to 900° C.

Advantageous Effects

When shape correction is performed for a hot-rolled coil manufactured bythe method for manufacturing the hot-rolled coil according to thepresent invention, it may delay the phase transformation of the steelduring cooling after hot rolling, thereby preventing deterioration inthe material and physical properties of the hot-rolled coil whileexhibiting an excellent effect of preventing deformation of thehot-rolled coil. In addition, the use of correction by self-weight andgravity makes it possible to prevent surface defects (such as scratches)of the hot-rolled coil, which occur when correction by an external forceis used. In addition, it may reduce the correction cost and provideexcellent economic efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a method for manufacturing a hot-rolled coil according toone embodiment of the present invention.

FIG. 2 shows a method for correcting the shape of a hot-rolled coilaccording to one embodiment of the present invention.

FIG. 3 schematically shows a method for correcting the shape of ahot-rolled coil according to one embodiment of the present invention.

FIG. 4(a) is a photograph showing a hot-rolled coil according to oneexample of the present invention immediately after coiling, and FIG.4(b) is a photograph showing the hot-rolled coil after air cooling.

FIG. 5(a) is a photograph showing a hot-rolled coil according to anotherexample of the present invention immediately after coiling, and FIG.5(b) is a photograph showing the hot-rolled coil after air cooling.

FIG. 6(a) is a photograph showing a hot-rolled coil of a comparativeexample for the present invention immediately after coiling, and FIG.6(b) is a photograph showing the hot-rolled coil after air cooling.

FIG. 7 is a graph comparing the phase transformation curves ofhot-rolled coils with the passage of hot-rolled coil manufacturing timeand shape correction time in an example of the present invention and acomparative example for the present invention.

MODE FOR INVENTION Detailed Description

Hereinafter, the present invention will be described in detail. In thefollowing description of the present invention, the detailed descriptionof related known technologies or configurations will be omitted when itmay unnecessarily obscure the subject matter of the present invention.

In addition, the terms used in the following description are defined inconsideration of their functions in the present invention and may varydepending on a user's or operator's intension or usual practice.Accordingly, the definition should be made based on the contents throughthe specification that describes the present invention.

Method for Manufacturing Hot-Rolled Coil

One aspect of the present invention is directed to a method formanufacturing a hot-rolled coil. FIG. 1 shows a method for manufacturinga hot-rolled coil according to one embodiment of the present invention.In one embodiment, the method for manufacturing the hot-rolled coilincludes the steps of: (S10) reheating a steel slab; (S20) hot rolling;and (S30) coiling. More specifically, the method for manufacturing thehot-rolled coil includes the steps of: (S10) reheating a steel slabincluding 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si),0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur(S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), morethan 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt %titanium (Ti), and the remainder being iron (Fe) and other inevitableimpurities; (S20) hot-rolling the steel slab at a finishing milldelivery temperature of 850° C. to 950° C., thereby forming a hot-rolledsheet; and (S30) cooling the hot-rolled sheet, followed by coiling at acoiling temperature of 700° C. or higher.

Hereinafter, each step of the method for manufacturing the hot-rolledcoil according to the present invention will be described in detail.

(S10) Steel Slab Reheating Step

This step is a step of reheating a steel slab including 0.18 to 0.56 wt% carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese(Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), morethan 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % butnot more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainderbeing iron (Fe) and other inevitable impurities.

Hereinafter, the components included in the steel slab will be describedin detail.

Carbon (C)

Carbon (C) is added to ensure strength. Carbon is contained in an amountof 0.18 to 0.56 wt % based on the total weight of the steel slab. If thecontent of carbon is less than 0.18 wt %, it may be difficult to ensuresufficient strength. On the other hand, if the content of carbon is morethan 0.56 wt %, toughness may be reduced.

Silicon (Si)

Silicon (Si) functions as a deoxidizer for removing oxygen from thesteel and is added for solid solution strengthening. In one embodiment,silicon is contained in an amount of 0.1 to 0.5 wt % based on the totalweight of the steel slab. If the content of silicon is less than 0.1 wt%, the effect of adding the same will be insufficient, and if thecontent of silicon is more than 0.5 wt %, it may reduce weldability andproduce red scale during reheating and hot rolling, thus adverselyaffecting the surface quality. In addition, it may adversely affect thecoating property after welding.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element which iseffective in ensuring strength by increasing the hardenability of thesteel. In addition, manganese is an austenite stabilizing element whichcontributes to ferrite grain refinement by delaying ferrite and pearlitetransformation.

In one embodiment, manganese is contained in 0.7 to 6.5 wt % based onthe total weight of the steel slab. If the content of manganese is lessthan 0.7 wt %, the solid solution strengthening effect may beinsufficient. On the other hand, the content of manganese is more than6.5 wt %, weldability may be greatly reduced. In addition, a problem mayarise in that the ductility of the steel sheet is greatly reduced due tothe formation of an MnS inclusion and the occurrence of centralsegregation.

Phosphorus (P)

Phosphorus (P) is added to inhibit cementite formation and increasestrength. However, phosphorus deteriorates weldability and causes adifference in final properties by slab center segregation. For thisreason, in the present invention, the content of phosphorus (P) islimited to more than 0 wt % but not more than 0.02 wt % based on thetotal weight of the steel slab.

Sulfur

Sulfur (S) is an element that reduces the toughness and weldability ofthe steel and binds to manganese to form a non-metallic inclusion (MnS)that causes cracks during processing of the steel. For this reason, thecontent of sulfur (S) is limited to more than 0 wt % but not more than0.02 wt % based on the total weight of the steel slab.

Chromium (Cr)

Chromium is added for the purpose of increasing the hardenability andstrength of the steel. In one embodiment, chromium is contained in anamount of more than 0 wt % but not more than 0.3 wt % based on the totalweight of the steel slab. If the content of chromium is more than 0.3 wt%, the toughness of the hot-rolled coil may be reduced.

Boron (B)

Boron (B) is added for the purpose of compensating for hardenability byreplacing the expensive hardening element molybdenum, and has the effectof refining grains by increasing the austenite grain growth temperature.

In one embodiment, boron is contained in an amount of more than 0 wt %but not more than 0.004 wt % based on the total weight of the steelslab. If boron is contained in an amount of more than 0.004 wt %, therisk of reducing elongation may increase.

Titanium (Ti)

Titanium (Ti) is added for the purpose of enhancing hardenability andimproving properties by precipitate formation. In addition, iteffectively contributes to austenite grain refinement by formingprecipitate phases such as Ti(C,N) at high temperature.

In one embodiment, titanium is contained in an amount of 0.01 to 0.04 wt% based on the total weight of the steel slab. If titanium is containedin an amount of less than 0.01 wt %, the effect of adding the same maybe insufficient, and if titanium is contained in an amount of more than0.04 wt %, continuous casting defects may occur, it may be difficult toensure the physical properties of the hot-rolled coil, and cracks on thesurface of the hot-rolled coil may occur.

The remainder other than the above-described components is substantiallycomposed of iron (Fe). As used herein, the expression “remainder issubstantially composed of iron (Fe)” means that one containing othertrace elements, including inevitable impurities, may be included in thepresent invention, as long as it does not impair the effect of thepresent invention.

In one embodiment, the steel slab may be applied to a medium-carbonhot-rolled coil. For example, the steel slab may include 0.21 to 0.37 wt% carbon (C), 0.1 to 0.4 wt % silicon (Si), 1.1 to 1.5 wt % manganese(Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), morethan 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 to 0.3 wt %chromium (Cr), 0.001 to 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium(Ti), and the remainder being iron (Fe) and other inevitable impurities.

In another embodiment, the steel slab may be applied to a high-manganesehot-rolled coil. For example, the steel slab may include 0.18 to 0.25 wt% carbon, 0.3 to 0.5 wt % silicon (Si), 2 to 6.5 wt % manganese (Mn),more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0wt % but not more than 0.01 wt % sulfur (S), more than 0 wt % but notmore than 0.1 wt % chromium (Cr), more than 0 wt % but not more than0.001 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainderbeing iron (Fe) and other inevitable impurities.

In still another embodiment, the steel slab may be applied to ahigh-carbon hot-rolled coil. For example, the steel slab may include 0.5to 0.56 wt % carbon (C), 0.1 to 0.3 wt % silicon (Si), 0.7 to 1 wt %manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus(P), more than 0 wt % but not more than 0.01 wt % sulfur (S), 0.1 to 0.3wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron(B), 0.01 to 0.02 wt % titanium (Ti), and the remainder being iron (Fe)and other inevitable impurities.

In one embodiment, the steel slab may be heated at a slab reheatingtemperature (SRT) of 1,150° C. to 1,250° C. At this slab reheatingtemperature, the effect of homogenizing the alloying elements may beexcellent.

(S20) Hot-Rolling Step

This step is a step of hot-rolling the steel slab at a finishing milldelivery temperature of 850° C. to 950° C., thereby forming a hot-rolledsheet. When hot rolling is performed at this finishing mill deliverytemperature, the hot-rolled coil may have both excellent rigidity andexcellent moldability and is excellent in terms of coiling workability,and the effect of preventing deformation of the hot-rolled coil may beexcellent.

(S30) Coiling Step

This step is a step of cooling the hot-rolled sheet, followed by coilingat a coiling temperature of 700° C. or higher. In one embodiment, thehot-rolled sheet may be cooled to the coiling temperature and coiled atthat temperature. In one embodiment, the cooling may be performed by aircooling without using cooling water. When the cooling is performed underthe above-described conditions, the occurrence of bulging defects on thehot-rolled coil may be effectively reduced. As used herein, the “bulgingdefects” may refer to shape distortion defects of the hot-rolled coil.Specifically, the “bulging defects” may refer to shape distortiondefects caused by the change of the inner and outer diameters of thehot-rolled coil to an ellipse rather than a circle, due to thedistortion of the hot-rolled coil in the direction of gravity, amongshape defects that occur on the hot-rolled coil.

After the sheet including the alloying components of the presentinvention is hot-rolled, the control of cooling may be performed suchthat the coiling is completed at a temperature equal to or higher than atemperature at which phase transformation begins. When coiling isperformed at the above-described coiling temperature, ferrite phasetransformation begins after a certain time after the coiling, and forthis reason, the time taken for phase transformation to be completed mayincrease rapidly due to slow cooling (air cooling) of the coil after thecoiling, thereby advantageously preventing shape deformation. Namely,one embodiment of the present invention may provide process conditionsthat delay the time point of occurrence of phase transformation aftercoiling as much as possible.

If the hot-rolled sheet is coiled at a coiling temperature lower than700° C., phase transformation of the hot-rolled sheet may proceed in thecooling process, and additional phase transformation may occur afterformation of the hot-rolled coil, resulting in an increase in the coilvolume, and then the hot-rolled coil may shrink with loweringtemperature and the shape thereof is deformed to an elliptical shape byself-weight, thus causing bulging defects. In one embodiment, thehot-rolled sheet may be cooled and coiled at a coiling temperature of700° C. to 900° C. For example, the coiling may be performed at acoiling temperature of 730° C. to 820° C. The manufactured hot-rolledcoil may include ferrite and bainite microstructures.

Method for Correcting Shape of Hot-Rolled Coil

Another aspect of the present invention is directed to a method forcorrecting the shape of a hot-rolled coil. FIG. 2 shows a method forcorrecting the shape of a hot-rolled coil according to one embodiment ofthe present invention. Referring to FIG. 2, the method for correctingthe shape of the hot-rolled coil includes the steps of: (S101) mountingthe hot-rolled coil; (S102) measuring the longest diameter of thehot-rolled coil; (S103) adjusting the position of the hot-rolled coil;and (S104) lifting.

FIG. 3 schematically shows a method for correcting the shape of ahot-rolled coil according to one embodiment of the present invention.Referring to FIG. 3, the method for correcting the shape of thehot-rolled coil includes the steps of: (S101) mounting the hot-rolledcoil on a hanger forming the lower part of a C-hook; (S102) measuringthe longest diameter of the hot-rolled coil using an outer diametermeasuring means provided in the upper part of the C-hook; (S103)adjusting the longest diameter of the hot-rolled coil to beperpendicular to the C-hook by means of a driving roll provided on thelower hanger; and (S104) placing the C-hook, which has the hot-rolledcoil mounted thereon, on a stand, followed by lifting, therebycorrecting the shape of the hot-rolled coil by self-weight.

For example, as shown in FIG. 3(a), a hot-rolled coil 100 is mounted ona hanger 201 forming the lower part of a C-hook 200. As shown in FIG.3(b), the longest diameter of the hot-rolled coil 100 is measured usingan outer diameter measuring means 210 provided in the upper part 202 ofthe C-hook. Next, as shown in FIG. 3(c), using a driving roll 220provided on the hanger 201 forming the lower part, the longest diameterof the hot-rolled coil 100 is adjusted perpendicular to the C-hook. Asshown in FIG. 3(e), the C-hook 200 having the hot-rolled coil mountedthereon is placed on a stand 300 and lifted, and thus as shown in FIG.3(f), the hot-rolled coil shape distorted into an elliptical shape maybe corrected into a circular shape by self-weight.

The hot-rolled coil is manufactured by a method including the steps of:reheating a steel slab including 0.18 to 0.56 wt % carbon (C), 0.1 to0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt %but not more than 0.02 wt % phosphorus (P), more than 0 wt % but notmore than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron(B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe)and other inevitable impurities; hot-rolling the steel slab at afinishing mill delivery temperature of 850° C. to 950° C., therebyforming a hot-rolled sheet; and cooling the hot-rolled sheet, followedby coiling at a coiling temperature of 700° C. or higher. In oneembodiment, the hot-rolled coil may be manufactured by cooling thehot-rolled sheet, followed by coiling at a coiling temperature of 700°C. to 900° C. The manufactured hot-rolled coil may include ferrite andbainite microstructures.

The method for manufacturing the hot-rolled coil may be performed usingthe same steel slab as used in the above-described method formanufacturing the hot-rolled coil, and thus the detailed descriptionthereof is omitted.

In one embodiment, the hot-rolled coil may be a medium-carbon hot-rolledmaterial. It may include 0.21 to 0.37 wt % carbon (C), 0.1 to 0.4 wt %silicon (Si), 1.1 to 1.5 wt % manganese (Mn), more than 0 wt % but notmore than 0.02 wt % phosphorus (P), more than 0 wt % but not more than0.02 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), 0.001 to 0.004 wt %boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron(Fe) and other inevitable impurities.

In another embodiment, the hot-rolled coil may be a high-manganesehot-rolled material. It may include 0.18 to 0.25 wt % carbon, 0.3 to 0.5wt % silicon (Si), 2 to 6.5 wt % manganese (Mn), more than 0 wt % butnot more than 0.02 wt % phosphorus (P), more than 0 wt % but not morethan 0.01 wt % sulfur (S), more than 0 wt % but not more than 0.1 wt %chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B),0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) andother inevitable impurities.

In still another embodiment, the hot-rolled coil may be a high-carbonhot-rolled material. It may include 0.5 to 0.56 wt % carbon (C), 0.1 to0.3 wt % silicon (Si), 0.7 to 1 wt % manganese (Mn), more than 0 wt %but not more than 0.02 wt % phosphorus (P), more than 0 wt % but notmore than 0.01 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), more than0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.02 wt %titanium (Ti), and the remainder being iron (Fe) and other inevitableimpurities.

When shape correction is performed for a hot-rolled coil manufactured bythe method for manufacturing the hot-rolled coil according to thepresent invention, it may prevent the phase transformation of the steelduring cooling after hot rolling, thereby preventing deterioration inthe material and physical properties of the hot-rolled coil whileexhibiting an excellent effect of preventing deformation of thehot-rolled coil. In addition, the use of correction by self-weight andgravity makes it possible to prevent surface defects (such as scratches)of the hot-rolled coil, which occur when correction by an external forceis used. In addition, it may exclude an existing correction apparatusemploying an external force, thus reducing the correction cost andproviding excellent economic efficiency.

Hereinafter, the constitution and effects of the present invention willbe described in more detail with reference to preferred examples.However, these examples are given merely as illustrative of the presentinvention and are not to be construed as limiting the scope of thepresent invention in any way.

EXAMPLES AND COMPARATIVE EXAMPLES Example 1

As a medium-carbon material, a steel slab including 0.23 wt % carbon(C), 0.2 wt % silicon (Si), 1.2 wt % manganese (Mn), 0.015 wt %phosphorus (P), 0.01 wt % sulfur (S), 0.2 wt % chromium (Cr), 0.003 wt %boron (B), 0.02 wt % titanium (Ti), and the remainder being iron (Fe)and other inevitable impurities, was reheated at 1200° C., and the steelslab was hot-rolled at a finishing mill delivery temperature of 880° C.,thereby forming a hot-rolled sheet. Then, the hot-rolled sheet wascooled and coiled at a coiling temperature of 700° C., therebymanufacturing a hot-rolled coil.

Examples 2

As a high-manganese material, a steel slab including 0.2 wt % carbon(C), 0.4 wt % silicon (Si), 6 wt % manganese (Mn), 0.015 wt % phosphorus(P), 0.01 wt % sulfur (S), 0.05 wt % chromium (Cr), 0.001 wt % boron(B), 0.02 wt % titanium (Ti), and the remainder being iron (Fe) andother inevitable impurities, was reheated at 1200° C., and the steelslab was hot-rolled at a finishing mill delivery temperature of 940° C.,thereby forming a hot-rolled sheet. Then, the hot-rolled sheet wascooled and coiled at a coiling temperature of 700° C., therebymanufacturing a hot-rolled coil.

Example 3

As a high-carbon material, a steel slab including 0.55 wt % carbon (C),0.2 wt % silicon (Si), 0.8 wt % manganese (Mn), 0.015 wt % phosphorus(P), 0.01 wt % sulfur (S), 0.2 wt % chromium (Cr), 0.001 wt % boron (B),0.01 wt % titanium (Ti), and the remainder being iron (Fe) and otherinevitable impurities, was reheated at 1200° C., and the steel slab washot-rolled at a finishing mill delivery temperature of 890° C., therebyforming a hot-rolled sheet. Then, the hot-rolled sheet was cooled andcoiled at a coiling temperature of 730° C., thereby manufacturing ahot-rolled coil.

Comparative Example 1

A hot-rolled coil was manufactured in the same manner as described inExample 1, except that the hot-rolled sheet was coiled at a coilingtemperature of 560° C.

Comparative Example 2

A hot-rolled coil was manufactured in the same manner as described inExample 1, except that the hot-rolled sheet was coiled at a coilingtemperature of 600° C.

Comparative Example 3

A hot-rolled coil was manufactured in the same manner as described inExample 1, except that the hot-rolled sheet was coiled at a coilingtemperature of 620° C.

Comparative Example 4

A hot-rolled coil was manufactured in the same manner as described inExample 1, except that the hot-rolled sheet was coiled at a coilingtemperature of 650° C.

FIG. 4(a) is a photograph showing the hot-rolled coil of Example 1according to the present invention immediately after coiling, and FIG.4(b) is a photograph showing the hot-rolled coil after air cooling. FIG.5(a) is a photograph showing the hot-rolled coil of Example 1 accordingto the present invention immediately after coiling, and FIG. 5(b) is aphotograph showing the hot-rolled coil after air cooling. FIG. 6(a) is aphotograph showing a hot-rolled coil of a comparative example for thepresent invention immediately after coiling, and FIG. 6(b) is aphotograph showing the hot-rolled coil after air cooling. Referring toFIGS. 4(a) and 4(b), in Example 1, bulging defects were not observedimmediately after coiling of the hot-rolled coil, but bulging defectswere observed after air cooling. However, it could be seen that thedegree of bulging defects was smaller than that in the ComparativeExample. Referring to FIGS. 5(a) and 5(b), in Example 2, bulging defectswere not observed immediately after coiling of the hot-rolled coil andafter air cooling. Referring to FIGS. 6(a) and 6(b), in the ComparativeExample, bulging defects were observed immediately after coiling of thehot-rolled coil, and it could be seen that the degree of the bulgingdefects become more severe as air cooling proceeded.

Correction of Shape of Hot-Rolled Coil

For the hot-rolled coils of Examples 1 to 3 and Comparative Examples 1to 4, shape correction was performed. Each of the hot-rolled coil wasmounted on a hanger forming the lower part of a C-hook, and the longestdiameter of the hot-rolled coil was measured using an outer diametermeasuring means provided on the upper part of the C-hook. Thereafter,using a driving roll provided on the hanger, the longest diameter of thehot-rolled coil was adjusted to be perpendicular to the C-hook. TheC-hook having the hot-rolled coil mounted thereon was placed on a standand lifted, thereby correcting the shape of the hot-rolled coil byself-weight.

For Examples 1 to 3 and Comparative Examples 1 to 4, the inner diameterof the coils and whether bulging defects would be corrected after shapecorrection were observed, and the results of the observation are shownin Table 1 below.

TABLE 1 Whether bulging Coiling Coil inner defects would be temperaturediameter corrected by shape (° C.) (mm) correction Example 1 700 740Corrected Example 2 730 760 Corrected Example 3 730 740 CorrectedComparative 560 700 Not corrected Example 1 Comparative 600 710 Notcorrected Example 2 Comparative 620 680 Not corrected Example 3Comparative 650 720 Not corrected Example 4

Referring to Table 1 above, it could be seen that, in the case ofExamples 1 to 3, bulging defects did not appear after correction, but inthe case of Comparative Examples 1 to 4 which were out of the coilingtemperature of the present invention, bulging defects were not properlycorrected even after correction.

FIG. 7 is a graph comparing the phase transformation curves ofhot-rolled coils with the passage of hot-rolled coil manufacturing timeand shape correction time in Example 1 and Comparative Example 1.Referring to FIG. 7, in the case of Example 1 of the present invention,in which a specific alloying element system was applied and coiling wasperformed at a temperature (700° C.) equal to or higher than the phasetransformation temperature, and thus phase transformation to ferriteproceeded after a certain time after manufacturing of the hot-rolledcoil, it could be seen that the time taken for phase transformation tobe completed increased rapidly due to slow cooling (air cooling) of thecoil after coiling, indicating that Example 1 was advantageous for shapecorrection. However, in the case of Comparative Example 1 in whichcoiling was performed at a temperature lower than the phasetransformation temperature of the hot-rolled sheet, it could be seenthat phase transformation to ferrite occurred earlier than in Example 1,making it difficult to ensure the time of start of the phasetransformation of the present invention, indicating that ComparativeExample 1 was disadvantageous for shape correction.

In addition, in accordance with the methods for manufacturing thehot-rolled coil and correcting the shape of the hot-rolled coilaccording to the present invention, the occurrence of bulging of thehot-rolled coil could be reduced, thereby reducing additional operationscaused by breakage of the inner coil part, delayed operation time,facility breakage, etc., which would occur due to the bulging coil in asubsequent correction process, thereby providing effects, includingincreased work efficiency, increased material quality, reduced rate ofoccurrence of defective products disposed of as scrap, etc.

Simple modifications or alterations of the present invention may beeasily made by those skilled in the art, and such modifications oralterations may be considered to be all included within the scope of thepresent invention.

1. A method for manufacturing a hot-rolled coil, the method comprisingthe steps of: reheating a steel slab to form a reheated steel slab, thesteel slab comprising 0.18 wt % to 0.56 wt % carbon (C), 0.1 wt % to 0.5wt % silicon (Si), 0.7 wt % to 6.5 wt % manganese (Mn), more than 0 wt %but not more than 0.02 wt % phosphorus (P), more than 0 wt % but notmore than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron(B), 0.01 wt % to 0.04 wt % titanium (Ti), and the remainder being iron(Fe) and other inevitable impurities; hot-rolling the reheated steelslab at a finishing mill delivery temperature of 850° C. to 950° C.,thereby forming a hot-rolled sheet; and cooling the hot-rolled sheet,followed by coiling at a coiling temperature of 700° C. or higher. 2.The method of claim 1, wherein the steel slab comprises 0.21 wt % to0.37 wt % carbon (C), 0.1 wt % to 0.4 wt % silicon (Si), 1.1 wt % to 1.5wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt %phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S),0.1 wt % to 0.3 wt % chromium (Cr), 0.001 wt % to 0.004 wt % boron (B),0.01 wt % to 0.04 wt % titanium (Ti), and the remainder being iron (Fe)and other inevitable impurities.
 3. The method of claim 1, wherein thesteel slab comprises 0.18 wt % to 0.25 wt % carbon, 0.3 wt % to 0.5 wt %silicon (Si), 2 wt % to 6.5 wt % manganese (Mn), more than 0 wt % butnot more than 0.02 wt % phosphorus (P), more than 0 wt % but not morethan 0.01 wt % sulfur (S), more than 0 wt % but not more than 0.1 wt %chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B),0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) andother inevitable impurities.
 4. The method of claim 1, wherein the steelslab comprises 0.5 wt % to 0.56 wt % carbon (C), 0.1 wt % to 0.3 wt %silicon (Si), 0.7 wt % to 1 wt % manganese (Mn), more than 0 wt % butnot more than 0.02 wt % phosphorus (P), more than 0 wt % but not morethan 0.01 wt % sulfur (S), 0.1 wt % to 0.3 wt % chromium (Cr), more than0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.02 wt %titanium (Ti), and the remainder being iron (Fe) and other inevitableimpurities.
 5. The method of claim 1, wherein the hot-rolled sheet iscooled and coiled at a coiling temperature of 700° C. to 900° C.
 6. Amethod for correcting a shape of a hot-rolled coil, the methodcomprising the steps of: mounting the hot-rolled coil on a hangerforming a lower part of a C-hook; measuring a longest diameter of thehot-rolled coil using an outer diameter measuring means provided in anupper part of the C-hook; adjusting the longest diameter of thehot-rolled coil to be perpendicular to the C-hook by means of a drivingroll provided on the hanger; and placing the C-hook, which has thehot-rolled coil mounted thereon, on a stand, followed by lifting,thereby correcting the shape of the hot-rolled coil by self-weight.
 7. Amethod for correcting a shape of a hot-rolled coil, the methodcomprising the steps of: mounting the hot-rolled coil on a hangerforming a lower part of a C-hook; measuring a longest diameter of thehot-rolled coil using an outer diameter measuring means provided in anupper part of the C-hook; adjusting the longest diameter of thehot-rolled coil to be perpendicular to the C-hook by means of a drivingroll provided on the hanger; and placing the C-hook, which has thehot-rolled coil mounted thereon, on a stand, followed by lifting,thereby correcting the shape of the hot-rolled coil by self-weight,wherein the hot-rolled coil is manufactured by a method comprising thesteps of: reheating a steel slab comprising 0.18 wt % to 0.56 wt %carbon (C), 0.1 wt % to 0.5 wt % silicon (Si), 0.7 wt % to 6.5 wt %manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus(P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % butnot more than 0.004 wt % boron (B), 0.01 wt % to 0.04 wt % titanium(Ti), and the remainder being iron (Fe) and other inevitable impurities;hot-rolling the steel slab at a finishing mill delivery temperature of850° C. to 950° C., thereby forming a hot-rolled sheet; and cooling thehot-rolled sheet, followed by coiling at a coiling temperature of 700°C. or higher.
 8. The method of claim 7, wherein the hot-rolled coilcomprises 0.21 wt % to 0.37 wt % carbon (C), 0.1 wt % to 0.4 wt %silicon (Si), 1.1 wt % to 1.5 wt % manganese (Mn), more than 0 wt % butnot more than 0.02 wt % phosphorus (P), more than 0 wt % but not morethan 0.02 wt % sulfur (S), 0.1 wt % to 0.3 wt % chromium (Cr), 0.001 wt% to 0.004 wt % boron (B), 0.01 wt % to 0.04 wt % titanium (Ti), and theremainder being iron (Fe) and other inevitable impurities.
 9. The methodof claim 7, wherein the hot-rolled sheet is cooled and coiled at acoiling temperature of 700° C. to 900° C.